This invention relates to sensor devices for detecting a parameter at a selected site, for outputting an indication as to whether the selected parameter is within an accepted range and for generating, on the same line, an indication as to the xe2x80x9chealthxe2x80x9d of the sensor device.
There are many applications where temperature, pressure, vibration or some other parameter of equipment, of a conduit containing a flowing gas, liquid or other fluid, a tank or other stationary fluid container, or some other element at a selected site must be monitored and certain action taken if the sensed parameter is determined to be outside of a selected range. Further, since the site where such monitoring is being performed may be hazardous or difficult to reach, it is desirable that information about the parameter being monitored at the site be transmitted to a central control station and that the station also receive an indication if there is a malfunction or failure at the sensor device so that remedial action may be taken.
Heretofore, some available devices have been relatively simple and inexpensive, but have not had the capability of providing a self-diagnostic indication of device health, have provided limited flexibility in adjusting parameters, doing maintenance checks and the like, and have not been adapted for remote readout. At the other end of the spectrum have been smart digital transmitters which enable remote sensing and process monitoring. These solid state devices, having no moving parts, are generally more accurate, flexible and reliable, and have more complex communication capability, but are also far more complex and costly. Other available devices suffer from similar problems.
A need therefore exists for a relatively simple and reliable sensor device which can provide critical information with minimal complexity to a remote site, preferably over a single wire pair. This information would include both important status of the selected parameter and reliable self-diagnostic indication of the sensor device health. This is particularly important in hazardous environments so that a problem or potential problem does not go undetected because of a defect in the sensor device performing monitoring of a parameter at the particular site.
In accordance with the above, this invention provides a sensor device which includes a sensor for detecting a selected parameter, such as pressure or temperature at a selected site. The device also includes a controller which receives indications from the sensor of the condition, or value, of the selected parameter. The controller then generates a selected steady state signal, or a non-steady state signal, for example a pulse signal, when the sensed parameter is within a first range, for example an acceptable or normal range, and generates the other of the signals when the parameter is in a second range, for example an unacceptable or alarm range.
For preferred embodiments, the steady state signal is generated when the parameter is within an acceptable or normal range and the non-steady state, or pulse signal, when the parameter is in an unacceptable or alarm range. There may also be at least one additional unacceptable range beyond the initial unacceptable range, each of which results in the controller generating a different non-steady state signal, for example a higher, or lower, frequency pulse signal. In addition, the controller generates a null output, i.e., no output (open), when the parameter value being sensed is beyond the range of the sensor (an overrange or underrange value), or where there is a defect in the sensor device, including a malfunction of the sensor, a plugged process port leading to the sensor, a controller malfunction, a loss of power at the sensor device, a cut or break in the signal line from the sensor device, a device switch malfunction or some other failure condition. The controller may provide for a deadband where the first and second ranges overlap, and where the controller continues to generate the signal it is generating when in the deadband region. This serves to reduce ambiguity about the selected parameter status.
For a preferred embodiment, the controller includes a watchdog timer which continues to generate a pulsed output only if it receives refresh signals from the controller at selected time intervals. The controller is designed so that it stops providing refresh signals to the watchdog timer in response to selected error conditions at the device. Thus, if there is a loss of power or the controller is malfunctioning, it will stop generating resets to the watchdog timer. When there is no pulsed output from the watchdog timer, the controller generates a null output indicating a sensor device malfunction. The controller is also programmed to generate a null output on the detection of other error conditions, such as a faulty sensor, a plugged process sensor port, a controller output error, a switch failure or the like.
The parameter being detected by the sensor normally has at least slight variations with time as a result for example of flow turbulence, variations in ambient temperature, humidity, and the like, and variations in electrical source, etc. The controller can monitor variations in parameters received from the sensor and can generate null outputs in response to the sensor not conforming to a predetermined profile. For example, if the output from the sensor remains substantially constant for a selected time interval, this may be an indication that either the sensor is malfunctioning or that the process sensor port is plugged. For a particular application, other variations in the profile of the sensor outputs over a given time interval may be indicative of other error conditions which the controller can be programmed to recognize and respond to. The device can also operate in a xe2x80x9clearn modexe2x80x9d wherein operation under normal conditions are monitored for selected time periods and utilized to create an application profile. This self learning mode could be used for, but is not limited to, learning the profile of a continuous or changing process. It could then use this self-learned profile to monitor parameters for acceptable and non-conforming performance and provide the appropriate status indication. An example might be to learn a parameter""s profile conditions for day or evening process operation.
To further assure proper operation of the device, the output from the controller to a control element switch may loop back to a controller input to be compared against the desired generated output. Any difference in the desired generated output and the looped or fed back output signal is indicative of a device malfunction and can result in a null or other failure output from the controller. The switch is operated in response to the controller detecting the sensed parameter being in the second or unacceptable range. The switch output can then be used to control an alarm or shutdown indicator or operate a suitable control element to restore the parameter to the first or desired range. Such a control element might for example be a valve to increase or decrease pressure, or a thermoelectric element to heat or cool the monitored parameter as appropriate. Depending on the parameter sensed, other suitable controls might also be employed. Where an additional range is recognized by the controller, operation in this range may result in an automatic shutdown of at least relevant portions of the monitored system or initiate other appropriate action.
The device also includes an output line or line pair (mode/health line) from the device to which the steady state signal, non-steady state signal, and the null or other failure output are applied. Where a control element such as a valve is operated, this separate output from the device is provided to indicate the state of such control element as well as information about the sensor device health.
An alternative embodiment, this output (mode/health) line uses four, or more, signal states to indicate two, or more, operating range conditions and two, or more, classes of fault or failure conditions. In this embodiment, the controller generates a non-steady state output signal of one frequency (or duty cycle) when the selected parameter is in an acceptable range and a second frequency (or duty cycle) signal when the selected parameter is in an unacceptable range. A constant xe2x80x9conxe2x80x9d steady state output signal indicates one class of fault conditions, such as a grounded or shorted connection, and a null or xe2x80x9copenxe2x80x9d output signal indicates a variety of other fault conditions such as a sensor device defect, sensor malfunction, plugged port, watchdog time-out, loss of power, broken signal line, output switch malfunction, etc.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment as illustrated in the accompanying drawings.